Memory devices or systems such as NAND flash memories are widely used in modern lives. Due to higher storage capacity and lower cost realized by technology scaling, the memory devices or systems have been applied to digital still cameras, Universal Serial Bus (USB) memories, memory cards, media players, cell phones including smart phones, netbooks, and so on. In some cases, users may frequently write and erase short sentences that occupy very small areas of the memory devices or systems, which may cause issues in erasing memory blocks.